The vergence eye movement system is responsible for the normal alignment of the visual axes during binocular viewing. Although misalignment of the eyes is the most common human oculomotor disorder, the neurophysiology of the vergence system has received little experimental attention. The proposed project is a comprehensive examination of the neural substrates of the vergence system using electrophysiological and anatomical techniques. Monkeys will be trained to make precise saccadic, smooth pursuit, and vergence movements on demand. The position of each eye in the orbit will be measured using the very accurate electromagnetic search coil technique. Recordings will be made from identified lateral and medial rectus motoneurons as well as identified abducens internuclear neurons to determine the vergence signals carried by these cells. An attempt will be made to determine if a connection exists between tonic convergence (TC) cells in the midbrain and medial rectus motoneurons (MRMN). TC cells carry a pure vergence signal which is precisely that required by MRMNs. These experiments will involve dual recording with cross-correlation, spike triggered averaging, and simultaneous stimulation and recording. Another experiment will test the hypothesis that the recently discovered vergence burst neurons provide an input to a vergence integrator and another experiment will study the neuronal substrate of a slow or tonic convergence mechanism. Finally, an intracellular horseradish peroxidase (HRP) experiment will investigate the morphology and connections of cells carrying a vergence signal.